Various materials have been known to be used for active materials in nonaqueous-system secondary batteries. Among the materials, lithium composite metallic oxides, which have a lamellar rock-salt structure and are expressed by a general formula, LiaNibCocMndDeOf (where 0.2≤“a”≤1, “b”+“c”+“d”+“e”=1, 0≤“e”<1, “D” is at least one element selected from the group consisting of Li, Fe, Cr, Cu, Zn, Ca, Mg, Zr, S, Si, Na, K and Al, and 1.7≤“f”≤2.1), have been used universally as active materials for lithium-ion secondary batteries.
However, when a lithium composite metallic oxide expressed by the aforementioned general formula is used as an active material in a high-capacity secondary battery driven or operated with a high voltage required for on-vehicle secondary battery, for instance, the lithium composite metallic oxide has been unable to keep the standard for satisfying a capacity maintained rate of the secondary battery, because the resistance of the material to the high voltage has been insufficient.
Consequently, investigations have been actively carried out recently to upgrade various materials to be used as active materials in the resistance to high voltage. In making the investigations, the following three methods have been proposed commonly.
1) doping an active material with an element of different species 2) forming a protective film on the surface of an active material
3) changing the composition of an active material in the superficial layer
The method according to above-mentioned 1), and an advantageous effect thereof are concretely explained below. Doping an active material with an element, such as Al or Zr, which has not been present in the active material, enables degradations of the active material accompanied by charging and discharging operations, namely, accompanied by the absorption and release of Li, to be inhibitable.
The method according to above-mentioned 2), and an advantageous effect thereof are concretely explained below. As following Patent Application Publication No. 1 discloses, making a protective film on the surface of an active material with a salt of phosphoric acid, and preventing the active material from contacting directly with an electrolytic solution enable degradations of the active material resulting primarily from contacting with the electrolytic solution to be inhibitable.
The method according to above-mentioned 3), and an advantageous effect thereof are concretely explained below. Following Patent Application Publication No. 2 discloses an active material with an increased Al composition in an obtainable superficial layer thereof by coating the active material on the surface with an Al compound and then heat treating the active material with the Al compound coated thereon.
Moreover, disclosures on crystalline heterogeneous strains in lithium composite metallic oxides are available in Patent Application Publication Nos. 3 through 6 mentioned below.
Patent Application Publication No. 3 sets forth controlling crystalline heterogeneous strains in a lithium composite metallic oxide during a 4-V-class charging/discharging cycle. The publication points out that, when crystalline heterogeneous strains in a lithium composite metallic oxide are low, namely, when the crystallinity is high, slight collapses in the crystal structure results in greatly hindering the diffusion of lithium ions at the time of battery reactions and thereby a capacity maintained rate becomes low. Accordingly, upgrading the capacity maintained rate at the time of a 4-V-class charging/discharging mode or operation has been sought for. Moreover, in Patent Publication Literature No. 3, crystalline heterogeneous strains in a lithium composite metallic oxide are controlled by adding an element of different species to the fundamental constituent elements of the lithium composite metallic oxide. Accordingly, the lithium composite metallic oxide has been feared of being declined in the capacity to such an extent that the different-species element is added. From a viewpoint of the capacity, not adding the different-species element to the lithium composite metallic oxide is more preferable.
Patent Application Publication No. 4 sets forth that, in a hexagonal rock-salt-type crystal structure, such strains occur as stretching in the c-axis direction because repulsion forces occur between the oxygen atoms. The publication points out that not only the strains have an influence on the diffusion distance of Li and the stabilization of crystal structure, but also the strains result in making a high-capacity positive-electrode active material excelling in the cyclic durability obtainable.
Patent Application Publication No. 5 sets forth that defects and strains in the crystal lattice of an active material relieve expansive or contractive stresses in the lattice accompanied by charging/discharging mode or operations and the relieved stresses result in improving the active material in the cyclic longevity.
Patent Application Publication No. 6 sets forth that, even when a secondary battery is charged with a charge cut-off voltage of from 4.2 V up to 4.5 V against the lithium potential, setting a c-axis variation rate of the positive-electrode active material at a predetermined value or less leads to making the secondary battery upgradeable considerably in the cyclability.